remove SkBlitMask::PlatformRowProcs

It always returns nullptr.

Change-Id: I8d24e0f771f292faba5192cbdd311544153d1f3a
Reviewed-on: https://skia-review.googlesource.com/c/170060
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
5 files changed